Since offset printing can easily make multicolor printing, offset printing is used for printing on papers such as art papers, fine papers (i.e., wood-free papers), coated papers, etc., and polyethylene terephthalate films, polyvinyl chloride films, etc.
A drying type offset ink is composed of the composition shown in following Table 1.
TABLE 1 ______________________________________ Basic Composition of Offset Printing Ink: ______________________________________ ##STR1## ______________________________________
Also, as such an offset ink, a universal type offset ink for pulp papers, pulp coated papers, etc., has been generally used. An example of the composition of the universal type offset ink is shown in Table 2 below.
TABLE 2 ______________________________________ Offset Printing Ink Composition (parts by weight): Offset Ink for Universal Type Non-Absorbing Offset Ink Materials ______________________________________ Pigment 15 to 50 (20) (25) Resin 25 to 30 (30) (30) Drying Oil 10 to 15 (10) (34) High-Boiling 20 to 45 (35) (5) Petroleum Series Solvent Dryer 0.5 to 2 (1) (2) Other 2 to 5 (4) (4) Additives Total 100 100 ______________________________________ The numerals in ( ) are an example of a commercially available article.
As the universal type offset ink, a quick drying ink using the drying oil compounded with a resin and a mineral oil (high-boiling petroleum series solvent) as the vehicle of the offset ink is used in order to shorten drying time.
However, if polyolefin films, synthetic papers composed of stretched polyolefin films containing an inorganic fine powder as described in JP-B-46-40794 (the term "JP-B" as used herein means an "examined Japanese patent publication"), U.S. Pat. No. 4,075,050, British Patent 1,384,556, JP-A-62-227933, JP-A-61-3748, and JP-A-60-79951 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), or coated papers formed by applying a pigment coated layer on these synthetic papers for improving the drying property of the ink are printed using the foregoing universal type offset ink, a so-called solvent attack that the polyolefin film is swelled with the solvent (in particular, a high-boiling petroleum series solvent such as a mineral oil, etc.) of the offset ink used for universal type offset printing to cause local unevenness on the printed film or to curl the whole film, whereby there is a difficulty for use practically such a universal type offset ink.
Accordingly, as an offset printing ink for a polyolefin film, a specific offset printing ink containing a less amount of a high-boiling petroleum series solvent is used for a non-absorbing material such as a polyolefin film (see Table 2 above).
As one of the methods of preventing swelling of a polyolefin film with a solvent (in particular, a high-boiling solvent) of the universal type offset printing ink, it is considered to form a coating agent (pigment coated) layer on both the surfaces of a polyolefin film.
However, since the content of the solvent in the universal type offset printing ink is from 45 to 70% by weight, the solvent is liable to cause the solvent attack even in the case of employing the foregoing method. Therefore, for sufficiently absorbing the solvent with the coating agent layers for preventing the occurrence of the solvent attack of the polyolefin film, it is necessary that the thickness of each of the coating agent layers on both the surfaces of the polyolefin film is at least 35 .mu.m. However, if the thickness of the coating agent layers on both the surfaces thereof is more than 70 .mu.m, it takes a long time to dry after coating and it increases the cost of the coated resin film. Thus, the foregoing method is not a profitable method.
In the case of carrying out offset printing using such a specific offset printing ink for a polyolefin film, it is difficult to control the balance of the amount of water and the amount of the ink as compared to the case of using the universal type offset printing ink and also the viscosity of the ink is changed on printing machine to reduce the workability. Thus, printers are unwilling to use the printing ink. Accordingly, since printers using the printing ink and ink makers are limited, it has been desired to develop a polyolefin film which can be applied for the universal type offset printing ink on.
That is, in general printers, offset printing is usually applied to a pulp series papers such as a fine paper and a coated paper using the universal type offset printing ink and in only the case of printing on a polyolefin film or a synthetic paper, a specific offset printing ink for such a non-absorbing material must be used.
Accordingly, because a lot of time and a lot of works are required for the change of the printing ink, general printers do not positively print onto polyolefinic films such as synthetic papers, which is one cause of preventing the propagation of polyolefin films or synthetic papers for offset printing.